Interpretation chemical facts

I have touched few items selected in the varied activity of group II not sufficient to give a balanced appraisal of the evolution and of the prospects of the quantum molecular meoiy addressed to interpret chemical facts, but sufficient, I hope, to show that there is here, after more than thirty years of activity, a noticeable momentum, and that in the foreseeable future there will be other important progresses. 

The concept of chemical periodicity is central to the study of inorganic chemistry. No other generalization rivals the periodic table of the elements in its ability to systematize and rationalize known chemical facts or to predict new ones and suggest fruitful areas for further study. Chemical periodicity and the periodic table now find their natural interpretation in the detailed electronic structure of the atom indeed, they played a major role at the turn of the century in elucidating the mysterious phenomena of radioactivity and the quantum effects which led ultimately to Bohr s theory of the hydrogen atom. Because of this central position it is perhaps not surprising that innumerable articles and books have been written on the subject since the seminal papers by Mendeleev in 1869, and some 700 forms of the periodic table (classified into 146 different types or subtypes) have been proposed. A brief historical survey of these developments is summarized in the Panel opposite. 

Coulson s generation of quantum theoretical chemists was struck by the fact that the mathematical physics of wave mechanics did not result in fundamental breakthroughs or discoveries in chemistry. As we have seen, Mulliken claimed that his initial work in quantum mechanics "interpreted," rather than "discovered," chemical facts. Alberte Pullman commented in 1970 ... 

Quantum chemistry aims to understand a large variety of chemical facts. In some systems an interesting feature was obtained whose study and whose application can help to reduce the computational effort considerably this is the transferability. Transfer-ability can be interpreted in several ways. The orbitals, on the one hand, may be considered transferable in the case when certain properties of these orbitals are close to each other to a certain extent (Rothenberg, 1971). The transferability of orbitals can be discussed directly on the other hand too. Orbitals of small molecules can be used for constructing the wave-function of related, larger molecules. This can be done with or without further optimizations. In this interpretation the orbitals are transferable if the molecular properties calculated with and without optimizations are close to each other (O Leary et al, 1975). The transferability of orbitals for cyclic hydrocarbons was discussed exhaustively (Edmiston et al., 1963). 

Although inland waters represent more transitory systems than the sea, equilibrium models are also useful here for interpreting observed facts. Even in highly dynamic systems, we can obtain some limits on the variation of the chemical composition, and we can speculate on the type of dissolved species and solid phases one may expect. 

Research is the orderly search for knowledge aimed at the discovery and interpretation of facts. It is, of course, the traditional source of new chemical products. 

The interminable discussions on the interpretation of quantum theory that followed the pioneering events are now considered to be of interest only to philosophers and historians, but not to physicists. In their view, finality had been reached on acceptance of the Copenhagen interpretation and the mathematical demonstration by John von Neumann of the impossibility of any alternative interpretation. The fact that theoretical chemists still have not managed to realize the initial promise of solving all chemical problems by quantum mechanics probably only means some lack of insight on the their part. 

Mathematical chemistry, the new challenging discipline of chemistry has established itself in recent years. Its main goal is to develop formal (mathematical) methods for chemical theory and (to some extent) for data analysis. Its history may be traced back to Caley s attempt, more than 100 years ago, to use the graph theoretical representation and interpretation of the chemical constitution of molecules for the enumeration of acyclic chemical structures. Graph theory and related areas of discrete mathematics are the main tools of qualitative theoretical treatment of chemistry [1,2]. However, attempts to contemplate connections between mathematics and chemistry and to predict new chemical facts with the help of formal mathematics have been scarce throughout the entire history of chemistry. 

We have written The Hydrogen Bond with emphasis upon the physical and chemical facts and with three primary goals in mind. The first is to compile and summarize these experimental facts, thus furnishing a basis for prediction and interpretation of hydrogen bond behavior in substances not yet studied. The second is to present a critical discussion of the present state of the theory of this bond and its effects on physical and chemical behaviors. The third is to present a bibliography (comprehensive through 1956) which will aid workers in locating relevant studies. 

Although it has been suggested that some of the reactions of 2-ami-noimidazole can best be explained by its existence as the imino tautomer in dimethylformamide, this result was based on incorrectly interpreted chemical evidence. In fact, a UV study of 2-amino-l-methyl-4,5-di-phenylimidazole provides convincing evidence for the amino form with Kj = 3 X 10". Calculations of heats of combustion show that in the 4-aminoimidazole 5-aminoimidazole equilibrium there is a slight predominance of the 4-amino form [a conclusion which is inconsistent with Charton s hypothesis since (7m(NH2) = —0.161]. The compounds, however, exist as amino rather than imino structures. ... 

While the collection of chemical facts continued to be enlarged by the artisan, these facts were interpreted by the philosophers— who also served as mathematicians, astronomers, anatomists, and physicists, as well as theologians and political theoreticians. In fact not until the 1800s did European scientists begin to think of their work as separate from that of philosophers. Though philosophy is common to all cultures, the most influential in the development of modern chemistry were the philosophers of Greece. These thinkers derived hypotheses about the nature of matter and material interactions that helped and hindered chemical developments over the next 2000 years. 

If the effects calculated there should grasp the known chemical facts in Uieir nature—and not merely as the result of lengthy calculations— we wish to find the concepts of chemistry in the quantum-mechanical description and establish a connection with the structure of atoms. In chemistry these have proven to be reliable even in complicated cases, as a guide through the plethora of possible compounds. We especially want to interpret the valence numbers of the homo-polar compoimds on the basis of the conceptual framework of quantum-mechanics, in a fashion similar to the one in which the polar valence numbers have been interpreted fruitfully, though not in all cases fully satisfactorily, with the pictures of Kossel ind Lewis. 

All facts considered up to now point in the direction that the double bond is responsible for this special behaviour. Before going into the quantum-mechanical interpretation of this peculiar behaviour of the double bond, we will discuss some arguments which lead to the conclusion that the explanation of the chemical facts... 

As several of the Gottingen physicists who were exposed to these ideas by Bohr s own lectures later commented, the work rested on a mixture of ad hoc arguments and chemical facts without any strict derivations from the principles of quantum theory to which Bohr frequently alluded. As Kragh writes, it was realized in 1922 that Bohr s theory was not deductive, although admittedly Bohr drew on the observed X-ray spectra of elements which were interpreted with the aid of quantum theory. 

We shall not be concerned here with the exact treatment of spectra by normal coordinate analysis 24, 29). Instead an attempt will be made to give some semiempirical rules, which will enable us to predict the correct number and types of vibrations in the infrared or Raman spectrum for a given symmetry. If the observed spectrum coincides with theoretical expectations, we can be reasonably sure that we predicted the correct symmetry. However, we cannot be absolutely certain, since one can never be sure that the number of frequencies found is not too low (or more seldom too high) for an assumed symmetry. It is not so much the correct application of the rules, but rather the interpretation of the experimental findings that is decisive. It is, therefore, in most cases involving hydrocarbon complexes, advisable and sometimes necessary to obtain additional evidence from measurements on deuterated compounds, from comparisons of the spectra of similar compounds, or from considerations of chemical facts, in order to make sure that the determination of a symmetry group is well-founded. 

With more than 30 million organic compounds now known and thousands more being created daily, naming them all is a real problem. Part of the problem is due to the sheer complexity of organic structures, but part is also due to the fact that chemical names have more than one purpose. For Chemical Abstracts Service (CAS), which catalogs and indexes the worldwide chemical literature, each compound must have only one correct name. It would be chaos if half the entries for CH3B1 were indexed under "M" for methyl bromide and half under "B" for bromomethane. Furthermore, a CAS name must be strictly systematic so that it can be assigned and interpreted by computers common names are not allowed. 

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